Of Particular SignificanceConversations About Science with Theoretical Physicist Matt Strassler2018-03-19T14:11:14Zhttp://profmattstrassler.com/feed/atom/WordPress.comMatt Strasslerhttps://profmattstrassler.wordpress.comhttp://profmattstrassler.com/?p=101982018-02-28T05:38:03Z2018-02-05T15:59:40ZContinue reading →]]>This week, the community of high-energy physicists — of those of us fascinated by particles, fields, strings, black holes, and the universe at large — is mourning the loss of one of the great theoretical physicists of our time, Joe Polchinski. It pains me deeply to write these words.

Everyone who knew him personally will miss his special qualities — his boyish grin, his slightly wicked sense of humor, his charming way of stopping mid-sentence to think deeply, his athleticism and friendly competitiveness. Everyone who knew his research will feel the absence of his particular form of genius, his exceptional insight, his unique combination of abilities, which I’ll try to sketch for you below. Those of us who were lucky enough to know him both personally and scientifically — well, we lose twice.

Polchinski — Joe, to all his colleagues — had one of those brains that works magic, and works magically. Scientific minds are as individual as personalities. Each physicist has a unique combination of talents and skills (and weaknesses); in modern lingo, each of us has a superpower or two. Rarely do you find two scientists who have the same ones.

Joe had several superpowers, and they were really strong. He had a tremendous knack for looking at old problems and seeing them in a new light, often overturning conventional wisdom or restating that wisdom in a new, clearer way. And he had prodigious technical ability, which allowed him to follow difficult calculations all the way to the end, on paths that would have deterred most of us.

One of the greatest privileges of my life was to work with Joe, not once but four times. I think I can best tell you a little about him, and about some of his greatest achievements, through the lens of that unforgettable experience.

[To my colleagues: this post was obviously written in trying circumstances, and it is certainly possible that my memory of distant events is foggy and in error. I welcome any corrections that you might wish to suggest.]

Our papers between 1999 and 2006 were a sequence of sorts, aimed at understanding more fully the profound connection between quantum field theory — the language of particle physics — and string theory — best-known today as a candidate for a quantum theory of gravity. In each of those papers, as in many thousands of others written after 1995, Joe’s most influential contribution to physics played a central role. This was the discovery of objects known as “D-branes”, which he found in the context of string theory. (The term is a generalization of the word `membrane’.)

I can already hear the polemical haters of string theory screaming at me. ‘A discovery in string theory,’ some will shout, pounding the table, ‘an untested and untestable theory that’s not even wrong, should not be called a discovery in physics.’ Pay them no mind; they’re not even close, as you’ll see by the end of my remarks.

The Great D-scovery

In 1989, Joe, working with two young scientists, Jin Dai and Rob Leigh, was exploring some details of string theory, and carrying out a little mathematical exercise. Normally, in string theory, strings are little lines or loops that are free to move around anywhere they like, much like particles moving around in this room. But in some cases, particles aren’t in fact free to move around; you could, for instance, study particles that are trapped on the surface of a liquid, or trapped in a very thin whisker of metal. With strings, there can be a new type of trapping that particles can’t have — you could perhaps trap one end, or both ends, of the string within a surface, while allowing the middle of the string to move freely. The place wherea string’s end may be trapped — whether a point, a line, a surface, or something more exotic in higher dimensions — is what we now call a “D-brane”. [The `D’ arises for uninteresting technical reasons.]

Joe and his co-workers hit the jackpot, but they didn’t realize it yet. What they discovered, in retrospect, was that D-branes are an automatic feature of string theory. They’re not optional; you can’t choose to study string theories that don’t have them. And they aren’t just surfaces or lines that sit still. They’re physical objects that can roam the world. They have mass and create gravitational effects. They can move around and scatter off each other. They’re just as real, and just as important, as the strings themselves!

It was as though Joe and his collaborators started off trying to understand why the chicken crossed the road, and ended up discovering the existence of bicycles, cars, trucks, buses, and jet aircraft. It was that unexpected, and that rich.

And yet, nobody, not even Joe and his colleagues, quite realized what they’d done. Rob Leigh, Joe’s co-author, had the office next to mine for a couple of years, and we wrote five papers together between 1993 and 1995. Yet I think Rob mentioned his work on D-branes to me just once or twice, in passing, and never explained it to me in detail. Their paper had less than twenty citations as 1995 began.

In 1995 the understanding of string theory took a huge leap forward. That was the moment when it was realized that all five known types of string theory are different sides of the same die — that there’s really only one string theory. A flood of papers appeared in which certain black holes, and generalizations of black holes — black strings, black surfaces, and the like — played a central role. The relations among these were fascinating, but often confusing.

And then, on October 5, 1995, a paper appeared that changed the whole discussion, forever. It was Joe, explaining D-branes to those of us who’d barely heard of his earlier work, and showing that many of these black holes, black strings and black surfaces were actually D-branes in disguise. His paper made everything clearer, simpler, and easier to calculate; it was an immediate hit. By the beginning of 1996 it had 50 citations; twelve months later, the citation count was approaching 300.

So what? Great for string theorists, but without any connection to experiment and the real world. What good is it to the rest of us? Patience. I’m just getting to that.

What’s it Got to Do With Nature?

Our current understanding of the make-up and workings of the universe is in terms of particles. Material objects are made from atoms, themselves made from electrons orbiting a nucleus; and the nucleus is made from neutrons and protons. We learned in the 1970s that protons and neutrons are themselves made from particles called quarks and antiquarks and gluons — specifically, from a “sea” of gluons and a few quark/anti-quark pairs, within which sit three additional quarks with no anti-quark partner… often called the `valence quarks’. We call protons and neutrons, and all other particles with three valence quarks, `baryons”. (Note that there are no particles with just one valence quark, or two, or four — all you get is baryons, with three.)

In the 1950s and 1960s, physicists discovered short-lived particles much like protons and neutrons, with a similar sea, but which contain one valence quark and one valence anti-quark. Particles of this type are referred to as “mesons”. I’ve sketched a typical meson and a typical baryon in Figure 2. (The simplest meson is called a “pion”; it’s the most common particle produced in the proton-proton collisions at the Large Hadron Collider.)

Fig. 2: Baryons (such as protons and neutrons) and mesons each contain a sea of gluons and quark-antiquark pairs; baryons have three unpaired “valence” quarks, while mesons have a valence quark and a valence anti-quark. (What determines whether a quark is valence or sea involves subtle quantum effects, not discussed here.)

But the quark/gluon picture of mesons and baryons, back in the late 1960s, was just an idea, and it was in competition with a proposal that mesons are little strings. These are not, I hasten to add, the “theory of everything” strings that you learn about in Brian Greene’s books, which are a billion billion times smaller than a proton. In a “theory of everything” string theory, often all the types of particles of nature, including electrons, photons and Higgs bosons, are tiny tiny strings. What I’m talking about is a “theory of mesons” string theory, a much less ambitious idea, in which only the mesons are strings. They’re much larger: just about as long as a proton is wide. That’s small by human standards, but immense compared to theory-of-everything strings.

Why did people think mesons were strings? Because there was experimental evidence for it! (Here’s another example.) And that evidence didn’t go away after quarks were discovered. Instead, theoretical physicists gradually understood why quarks and gluons might produce mesons that behave a bit like strings. If you spin a meson fast enough (and this can happen by accident in experiments), its valence quark and anti-quark may separate, and the sea of objects between them forms what is called a “flux tube.” See Figure 3. [In certain superconductors, somewhat similar flux tubes can trap magnetic fields.] It’s kind of a thick string rather than a thin one, but still, it shares enough properties with a string in string theory that it can produce experimental results that are similar to string theory’s predictions.

Fig. 3: One reason mesons behave like strings in experiment is that a spinning meson acts like a thick string, with the valence quark and anti-quark at the two ends.

And so, from the mid-1970s onward, people were confident that quantum field theories like the one that describes quarks and gluons can create objects with stringy behavior. A number of physicists — including some of the most famous and respected ones — made a bolder, more ambitious claim: that quantum field theory and string theory are profoundly related, in some fundamental way. But they weren’t able to be precise about it; they had strong evidence, but it wasn’t ever entirely clear or convincing.

In particular, there was an important unresolved puzzle. If mesons are strings, then what are baryons? What are protons and neutrons, with their three valence quarks? What do they look like if you spin them quickly? The sketches people drew looked something like Figure 3. A baryon would perhaps become three joined flux tubes (with one possibly much longer than the other two), each with its own valence quark at the end. In a stringy cartoon, that baryon would be three strings, each with a free end, with the strings attached to some sort of junction. This junction of three strings was called a “baryon vertex.” If mesons are little strings, the fundamental objects in a string theory, what is the baryon vertex from the string theory point of view?! Where is it hiding — what is it made of — in the mathematics of string theory?

Fig. 4: A fast-spinning baryon looks vaguely like the letter Y — three valence quarks connected by flux tubes to a “baryon vertex”. A cartoon of how this would appear from a stringy viewpoint, analogous to Fig. 3, leads to a mystery: what, in string theory, is this vertex?!

[Experts: Notice that the vertex has nothing to do with the quarks. It’s a property of the sea — specifically, of the gluons. Thus, in a world with only gluons — a world whose strings naively form loops without ends — it must still be possible, with sufficient energy, to create a vertex-antivertex pair. Thus field theory predicts that these vertices must exist in closed string theories, though they are linearly confined.]

The baryon puzzle: what is a baryon from the string theory viewpoint?

No one knew. But isn’t it interesting that the most prominent feature of this vertex is that it is a location where a string’s end can be trapped?

Everything changed in the period 1997-2000. Following insights from many other physicists, and using D-branes as the essential tool, Juan Maldacena finally made the connection between quantum field theory and string theory precise. He was able to relate strings with gravity and extra dimensions, which you can read about in Brian Greene’s books, with the physics of particles in just three spatial dimensions, similar to those of the real world, with only non-gravitational forces. It was soon clear that the most ambitious and radical thinking of the ’70s was correct — that almost every quantum field theory, with its particles and forces, can alternatively be viewed as a string theory. It’s a bit analogous to the way that a painting can be described in English or in Japanese — fields/particles and strings/gravity are, in this context, two very different languages for talking about exactly the same thing.

The saga of the baryon vertex took a turn in May 1998, when Ed Witten showed how a similar vertex appears in Maldacena’s examples. [Note added: I had forgotten that two days after Witten’s paper, David Gross and Hirosi Ooguri submitted a beautiful, wide-ranging paper, whose section on baryons contains many of the same ideas.] Not surprisingly, this vertex was a D-brane — specifically a D-particle, an object on which the strings extending from freely-moving quarks could end. It wasn’t yet quite satisfactory, because the gluons and quarks in Maldacena’s examples roam free and don’t form mesons or baryons. Correspondingly the baryon vertex isn’t really a physical object; if you make one, it quickly diffuses away into nothing. Nevertheless, Witten’s paper made it obvious what was going on. To the extent real-world mesons can be viewed as strings, real-world protons and neutrons can be viewed as strings attached to a D-brane.

The baryon puzzle, resolved. A baryon is made from three strings and a point-like D-brane. [Note there is yet another viewpoint in which a baryon is something known as a skyrmion, a soliton made from meson fields — but that is an issue for another day.]

It didn’t take long for more realistic examples, with actual baryons, to be found by theorists. I don’t remember who found one first, but I do know that one of the earliest examples showed up in my first paper with Joe, in the year 2000.

Working with Joe

That project arose during my September 1999 visit to the KITP (Kavli Institute for Theoretical Physics) in Santa Barbara, where Joe was a faculty member. Some time before that I happened to have studied a field theory (called N=1*) that differed from Maldacena’s examples only slightly, but in which meson-like objects do form. One of the first talks I heard when I arrived at KITP was by Rob Myers, about a weird property of D-branes that he’d discovered. During that talk I made a connection between Myers’ observation and a feature of the N=1* field theory, and I had one of those “aha” moments that physicists live for. I suddenly knew what the string theory that describes the N=1* field theory must look like.

But for me, the answer was bad news. To work out the details was clearly going to require a very difficult set of calculations, using aspects of string theory about which I knew almost nothing [non-holomorphic curved branes in high-dimensional curved geometry.] The best I could hope to do, if I worked alone, would be to write a conceptual paper with lots of pictures, and far more conjectures than demonstrable facts.

But I was at KITP. Joe and I had had a good personal rapport for some years, and I knew that we found similar questions exciting. And Joe was the brane-master; he knew everything about D-branes. So I decided my best hope was to persuade Joe to join me. I engaged in a bit of persistent cajoling. Very fortunately for me, it paid off.

I went back to the east coast, and Joe and I went to work. Every week or two Joe would email some research notes with some preliminary calculations in string theory. They had such a high level of technical sophistication, and so few pedagogical details, that I felt like a child; I could barely understand anything he was doing. We made slow progress. Joe did an important warm-up calculation, but I found it really hard to follow. If the warm-up string theory calculation was so complex, had we any hope of solving the full problem? Even Joe was a little concerned.

And then one day, I received a message that resounded with a triumphant cackle — a sort of “we got ’em!” that anyone who knew Joe will recognize. Through a spectacular trick, he’d figured out how use his warm-up example to make the full problem easy! Instead of months of work ahead of us, we were essentially done.

From then on, it was great fun! Almost every week had the same pattern. I’d be thinking about a quantum field theory phenomenon that I knew about, one that should be visible from the string viewpoint — such as the baryon vertex. I knew enough about D-branes to develop a heuristic argument about how it should show up. I’d call Joe and tell him about it, and maybe send him a sketch. A few days later, a set of notes would arrive by email, containing a complete calculation verifying the phenomenon. Each calculation was unique, a little gem, involving a distinctive investigation of exotically-shaped D-branes sitting in a curved space. It was breathtaking to witness the speed with which Joe worked, the breadth and depth of his mathematical talent, and his unmatched understanding of these branes.

[Experts: It’s not instantly obvious that the N=1* theory has physical baryons, but it does; you have to choose the right vacuum, where the theory is partially Higgsed and partially confining. Then to infer, from Witten’s work, what the baryon vertex is, you have to understand brane crossings (which I knew about from Hanany-Witten days): Witten’s D5-brane baryon vertex operator creates a physical baryon vertex in the form of a D3-brane 3-ball, whose boundary is an NS 5-brane 2-sphere located at a point in the usual three dimensions. And finally, a physical baryon is a vertex with n strings that are connected to nearby D5-brane 2-spheres. See chapter VI, sections B, C, and E, of our paper from 2000.]

Throughout our years of collaboration, it was always that way when we needed to go head-first into the equations; Joe inevitably left me in the dust, shaking my head in disbelief. That’s partly my weakness… I’m pretty average (for a physicist) when it comes to calculation. But a lot of it was Joe being so incredibly good at it.

Fortunately for me, the collaboration was still enjoyable, because I was almost always able to keep pace with Joe on the conceptual issues, sometimes running ahead of him. Among my favorite memories as a scientist are moments when I taught Joe something he didn’t know; he’d be silent for a few seconds, nodding rapidly, with an intent look — his eyes narrow and his mouth slightly open — as he absorbed the point. “Uh-huh… uh-huh…”, he’d say.

But another side of Joe came out in our second paper. As we stood chatting in the KITP hallway, before we’d even decided exactly which question we were going to work on, Joe suddenly guessed the answer! And I couldn’t get him to explain which problem he’d solved, much less the solution, for several days!! It was quite disorienting.

This was another classic feature of Joe. Often he knew he’d found the answer to a puzzle (and he was almost always right), but he couldn’t say anything comprehensible about it until he’d had a few days to think and to turn his ideas into equations. During our collaboration, this happened several times. (I never said “Use your words, Joe…”, but perhaps I should have.) Somehow his mind was working in places that language doesn’t go, in ways that none of us outside his brain will ever understand. In him, there was something of an oracle.

Looking Toward The Horizon

Our interests gradually diverged after 2006; I focused on the Large Hadron Collider [also known as the Large D-brane Collider], while Joe, after some other explorations, ended up thinking about black hole horizons and the information paradox. But I enjoyed his work from afar, especially when, in 2012, Joe and three colleagues (Ahmed Almheiri, Don Marolf, and James Sully) blew apart the idea of black hole complementarity, widely hoped to be the solution to the paradox. [I explained this subject here, and also mentioned a talk Joe gave about it here.] The wreckage is still smoldering, and the paradox remains.

Then Joe fell ill, and we began to lose him, at far too young an age. One of his last gifts to us was his memoirs, which taught each of us something about him that we didn’t know. Finally, on Friday last, he crossed the horizon of no return. If there’s no firewall there, he knows it now.

What, we may already wonder, will Joe’s scientific legacy be, decades from now? It’s difficult to foresee how a theorist’s work will be viewed a century hence; science changes in unexpected ways, and what seems unimportant now may become central in future… as was the path for D-branes themselves in the course of the 1990s. For those of us working today, D-branes in string theory are clearly Joe’s most important discovery — though his contributions to our understanding of black holes, cosmic strings, and aspects of field theory aren’t soon, if ever, to be forgotten. But who knows? By the year 2100, string theory may be the accepted theory of quantum gravity, or it may just be a little-known tool for the study of quantum fields.

Yet even if the latter were to be string theory’s fate, I still suspect it will be D-branes that Joe is remembered for. Because — as I’ve tried to make clear — they’re real. Really real. There’s one in every proton, one in every neutron. Our bodies contain them by the billion billion billions. For that insight, that elemental contribution to human knowledge, our descendants can blame Joseph Polchinski.

Thanks for everything, Joe. We’ll miss you terribly. You so often taught us new ways to look at the world — and even at ourselves.

]]>29Matt Strasslerhttps://profmattstrassler.wordpress.comhttp://profmattstrassler.com/?p=100112017-10-17T22:14:32Z2017-10-17T16:03:35ZContinue reading →]]>Yesterday’s post on the results from the LIGO/VIRGO network of gravitational wave detectors was aimed at getting information out, rather than providing the pedagogical backdrop. Today I’m following up with a post that attempts to answer some of the questions that my readers and my personal friends asked me. Some wanted to understand better how to visualize what had happened, while others wanted more clarity on why the discovery was so important. So I’ve put together a post which (1) explains what neutron stars and black holes are and what their mergers are like, (2) clarifies why yesterday’s announcement was important — and there were many reasons, which is why it’s hard to reduce it all to a single soundbite. And (3) there are some miscellaneous questions at the end.

First, a disclaimer: I am *not* an expert in the very complex subject of neutron star mergers and the resulting explosions, called kilonovas. These are much more complicated than black hole mergers. I am still learning some of the details. Hopefully I’ve avoided errors, but you’ll notice a few places where I don’t know the answers … yet. Perhaps my more expert colleagues will help me fill in the gaps over time.

Please, if you spot any errors, don’t hesitate to comment!! And feel free to ask additional questions whose answers I can add to the list.

BASIC QUESTIONS ABOUT NEUTRON STARS, BLACK HOLES, AND MERGERS

What are neutron stars and black holes, and how are they related?

Every atom is made from a tiny atomic nucleus, made of neutrons and protons (which are very similar), and loosely surrounded by electrons. Most of an atom is empty space, so it can, under extreme circumstances, be crushed — but only if every electron and proton convert to a neutron (which remains behind) and a neutrino (which heads off into outer space.) When a giant star runs out of fuel, the pressure from its furnace turns off, and it collapses inward under its own weight, creating just those extraordinary conditions in which the matter can be crushed. Thus: a star’s interior, with a mass one to several times the Sun’s mass, is all turned into a several-mile(kilometer)-wide ball of neutrons — the number of neutrons approaching a 1 with 57 zeroes after it.

If the star is big but not too big, the neutron ball stiffens and holds its shape, and the star explodes outward, blowing itself to pieces in a what is called a core-collapse supernova. The ball of neutrons remains behind; this is what we call a neutron star. It’s a ball of the densest material that we know can exist in the universe — a pure atomic nucleus many miles(kilometers) across. It has a very hard surface; if you tried to go inside a neutron star, your experience would be a lot worse than running into a closed door at a hundred miles per hour.

If the star is very big indeed, the neutron ball that forms may immediately (or soon) collapse under its own weight, forming a black hole. A supernova may or may not result in this case; the star might just disappear. A black hole is very, very different from a neutron star. Black holes are what’s left when matter collapses irretrievably upon itself under the pull of gravity, shrinking down endlessly. While a neutron star has a surface that you could smash your head on, a black hole has no surface — it has an edge that is simply a point of no return, called a horizon. In Einstein’s theory, you can just go right through, as if passing through an open door. You won’t even notice the moment you go in. [Note: this is true in Einstein’s theory. But there is a big controversy as to whether the combination of Einstein’s theory with quantum physics changes the horizon into something novel and dangerous to those who enter; this is known as the firewall controversy, and would take us too far afield into speculation.] But once you pass through that door, you can never return.

Black holes can form in other ways too, but not those that we’re observing with the LIGO/VIRGO detectors.

Why are their mergers the best sources for gravitational waves?

One of the easiest and most obvious ways to make gravitational waves is to have two objects orbiting each other. If you put your two fists in a pool of water and move them around each other, you’ll get a pattern of water waves spiraling outward; this is in rough (very rough!) analogy to what happens with two orbiting objects, although, since the objects are moving in space, the waves aren’t in a material like water. They are waves in space itself.

To get powerful gravitational waves, you want objects each with a very big mass that are orbiting around each other at very high speed. To get the fast motion, you need the force of gravity between the two objects to be strong; and to get gravity to be as strong as possible, you need the two objects to be as close as possible (since, as Isaac Newton already knew, gravity between two objects grows stronger when the distance between them shrinks.) But if the objects are large, they can’t get too close; they will bump into each other and merge long before their orbit can become fast enough. So to get a really fast orbit, you need two relatively small objects, each with a relatively big mass — what scientists refer to as compact objects. Neutron stars and black holes are the most compact objects we know about. Fortunately, they do indeed often travel in orbiting pairs, and do sometimes, for a very brief period before they merge, orbit rapidly enough to produce gravitational waves that LIGO and VIRGO can observe.

Why do we find these objects in pairs in the first place?

Stars very often travel in pairs… they are called binary stars. They can start their lives in pairs, forming together in large gas clouds, or even if they begin solitary, they can end up pairing up if they live in large densely packed communities of stars where it is common for multiple stars to pass nearby. Perhaps surprisingly, their pairing can survive the collapse and explosion of either star, leaving two black holes, two neutron stars, or one of each in orbit around one another.

What happens when these objects merge?

Not surprisingly, there are three classes of mergers which can be detected: two black holes merging, two neutron stars merging, and a neutron star merging with a black hole. The first class was observed in 2015 (and announced in 2016), the second was announced yesterday, and it’s a matter of time before the third class is observed. The two objects may orbit each other for billions of years, very slowly radiating gravitational waves (an effect observed in the 70’s, leading to a Nobel Prize) and gradually coming closer and closer together. Only in the last day of their lives do their orbits really start to speed up. And just before these objects merge, they begin to orbit each other once per second, then ten times per second, then a hundred times per second. Visualize that if you can: objects a few dozen miles (kilometers) across, a few miles (kilometers) apart, each with the mass of the Sun or greater, orbiting each other 100 times each second. It’s truly mind-boggling — a spinning dumbbell beyond the imagination of even the greatest minds of the 19th century. I don’t know any scientist who isn’t awed by this vision. It all sounds like science fiction. But it’s not.

How do we know this isn’t science fiction?

We know, if we believe Einstein’s theory of gravity (and I’ll give you a very good reason to believe in it in just a moment.) Einstein’s theory predicts that such a rapidly spinning, large-mass dumbbell formed by two orbiting compact objects will produce a telltale pattern of ripples in space itself — gravitational waves. That pattern is both complicated and precisely predicted. In the case of black holes, the predictions go right up to and past the moment of merger, to the ringing of the larger black hole that forms in the merger. In the case of neutron stars, the instants just before, during and after the merger are more complex and we can’t yet be confident we understand them, but during tens of seconds before the merger Einstein’s theory is very precise about what to expect. The theory further predicts how those ripples will cross the vast distances from where they were created to the location of the Earth, and how they will appear in the LIGO/VIRGO network of three gravitational wave detectors. The prediction of what to expect at LIGO/VIRGO thus involves not just one prediction but many: the theory is used to predict the existence and properties of black holes and of neutron stars, the detailed features of their mergers, the precise patterns of the resulting gravitational waves, and how those gravitational waves cross space. That LIGO/VIRGO have detected the telltale patterns of these gravitational waves. That these wave patterns agree with Einstein’s theory in every detail is the strongest evidence ever obtained that there is nothing wrong with Einstein’s theory when used in these combined contexts. That then in turn gives us confidence that our interpretation of the LIGO/VIRGO results is correct, confirming that black holes and neutron stars really exist and really merge. (Notice the reasoning is slightly circular… but that’s how scientific knowledge proceeds, as a set of detailed consistency checks that gradually and eventually become so tightly interconnected as to be almost impossible to unwind. Scientific reasoning is not deductive; it is inductive. We do it not because it is logically ironclad but because it works so incredibly well — as witnessed by the computer, and its screen, that I’m using to write this, and the wired and wireless internet and computer disk that will be used to transmit and store it.)

THE SIGNIFICANCE(S) OF YESTERDAY’S ANNOUNCEMENT OF A NEUTRON STAR MERGER

What makes it difficult to explain the significance of yesterday’s announcement is that it consists of many important results piled up together, rather than a simple takeaway that can be reduced to a single soundbite. (That was also true of the black hole mergers announcement back in 2016, which is why I wrote a long post about it.)

So here is a list of important things we learned. No one of them, by itself, is earth-shattering, but each one is profound, and taken together they form a major event in scientific history.

First confirmed observation of a merger of two neutron stars: We’ve known these mergers must occur, but there’s nothing like being sure. And since these things are too far away and too small to see in a telescope, the only way to be sure these mergers occur, and to learn more details about them, is with gravitational waves. We expect to see many more of these mergers in coming years as gravitational wave astronomy increases in its sensitivity, and we will learn more and more about them.

New information about the properties of neutron stars: Neutron stars were proposed almost a hundred years ago and were confirmed to exist in the 60’s and 70’s. But their precise details aren’t known; we believe they are like a giant atomic nucleus, but they’re so vastly larger than ordinary atomic nuclei that can’t be sure we understand all of their internal properties, and there are debates in the scientific community that can’t be easily answered… until, perhaps, now.

From the detailed pattern of the gravitational waves of this one neutron star merger, scientists already learn two things. First, we confirm that Einstein’s theory correctly predicts the basic pattern of gravitational waves from orbiting neutron stars, as it does for orbiting and merging black holes. Unlike black holes, however, there are more questions about what happens to neutron stars when they merge. The question of what happened to this pair after they merged is still out — did the form a neutron star, an unstable neutron star that, slowing its spin, eventually collapsed into a black hole, or a black hole straightaway?

But something important was already learned about the internal properties of neutron stars. The stresses of being whipped around at such incredible speeds would tear you and I apart, and would even tear the Earth apart. We know neutron stars are much tougher than ordinary rock, but how much more? If they were too flimsy, they’d have broken apart at some point during LIGO/VIRGO’s observations, and the simple pattern of gravitational waves that was expected would have suddenly become much more complicated. That didn’t happen until perhaps just before the merger. So scientists can use the simplicity of the pattern of gravitational waves to infer some new things about how stiff and strong neutron stars are. More mergers will improve our understanding. Again, there is no other simple way to obtain this information.

First visual observation of an event that produces both immense gravitational waves and bright electromagnetic waves: Black hole mergers aren’t expected to create a brilliant light display, because, as I mentioned above, they’re more like open doors to an invisible playground than they are like rocks, so they merge rather quietly, without a big bright and hot smash-up. But neutron stars are big balls of stuff, and so the smash-up can indeed create lots of heat and light of all sorts, just as you might naively expect. By “light” I mean not just visible light but all forms of electromagnetic waves, at all wavelengths (and therefore at all frequencies.) Scientists divide up the range of electromagnetic waves into categories. These categories are radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays, listed from lowest frequency and largest wavelength to highest frequency and smallest wavelength. (Note that these categories and the dividing lines between them are completely arbitrary, but the divisions are useful for various scientific purposes. The only fundamental difference between yellow light, a radio wave, and a gamma ray is the wavelength and frequency; otherwise they’re exactly the same type of thing, a wave in the electric and magnetic fields.)

So if and when two neutron stars merge, we expect both gravitational waves and electromagnetic waves, the latter of many different frequencies created by many different effects that can arise when two huge balls of neutrons collide. But just because we expect them doesn’t mean they’re easy to see. These mergers are pretty rare — perhaps one every hundred thousand years in each big galaxy like our own — so the ones we find using LIGO/VIRGO will generally be very far away. If the light show is too dim, none of our telescopes will be able to see it.

But this light show was plenty bright. Gamma ray detectors out in space detected it instantly, confirming that the gravitational waves from the two neutron stars led to a collision and merger that produced very high frequency light. Already, that’s a first. It’s as though one had seen lightning for years but never heard thunder; or as though one had observed the waves from hurricanes for years but never observed one in the sky. Seeing both allows us a whole new set of perspectives; one plus one is often much more than two.

Over time — hours and days — effects were seen in visible light, ultraviolet light, infrared light, X-rays and radio waves. Some were seen earlier than others, which itself is a story, but each one contributes to our understanding of what these mergers are actually like.

Confirmation of the best guess concerning the origin of “short” gamma ray bursts: For many years, bursts of gamma rays have been observed in the sky. Among them, there seems to be a class of bursts that are shorter than most, typically lasting just a couple of seconds. They come from all across the sky, indicating that they come from distant intergalactic space, presumably from distant galaxies. Among other explanations, the most popular hypothesis concerning these short gamma-ray bursts has been that they come from merging neutron stars. The only way to confirm this hypothesis is with the observation of the gravitational waves from such a merger. That test has now been passed; it appears that the hypothesis is correct. That in turn means that we have, for the first time, both a good explanation of these short gamma ray bursts and, because we know how often we observe these bursts, a good estimate as to how often neutron stars merge in the universe.

First distance measurement to a source using both a gravitational wave measure and a redshift in electromagnetic waves, allowing a new calibration of the distance scale of the universe and of its expansion rate: The pattern over time of the gravitational waves from a merger of two black holes or neutron stars is complex enough to reveal many things about the merging objects, including a rough estimate of their masses and the orientation of the spinning pair relative to the Earth. The overall strength of the waves, combined with the knowledge of the masses, reveals how far the pair is from the Earth. That by itself is nice, but the real win comes when the discovery of the object using visible light, or in fact any light with frequency below gamma-rays, can be made. In this case, the galaxy that contains the neutron stars can be determined.

Once we know the host galaxy, we can do something really important. We can, by looking at the starlight, determine how rapidly the galaxy is moving away from us. For distant galaxies, the speed at which the galaxy recedes should be related to its distance because the universe is expanding.

How rapidly the universe is expanding has been recently measured with remarkable precision, but the problem is that there are two different methods for making the measurement, and they disagree. This disagreement is one of the most important problems for our understanding of the universe. Maybe one of the measurement methods is flawed, or maybe — and this would be much more interesting — the universe simply doesn’t behave the way we think it does.

What gravitational waves do is give us a third method: the gravitational waves directly provide the distance to the galaxy, and the electromagnetic waves directly provide the speed of recession. There is no other way to make this type of joint measurement directly for distant galaxies. The method is not accurate enough to be useful in just one merger, but once dozens of mergers have been observed, the average result will provide important new information about the universe’s expansion. When combined with the other methods, it may help resolve this all-important puzzle.

Best test so far of Einstein’s prediction that the speed of light and the speed of gravitational waves are identical: Since gamma rays from the merger and the peak of the gravitational waves arrived within two seconds of one another after traveling 130 million years — that is, about 5 thousand million million seconds — we can say that the speed of light and the speed of gravitational waves are both equal to the cosmic speed limit to within one part in 2 thousand million million. Such a precise test requires the combination of gravitational wave and gamma ray observations.

Efficient production of heavy elements confirmed: It’s long been said that we are star-stuff, or stardust, and it’s been clear for a long time that it’s true. But there’s been a puzzle when one looks into the details. While it’s known that all the chemical elements from hydrogen up to iron are formed inside of stars, and can be blasted into space in supernova explosions to drift around and eventually form planets, moons, and humans, it hasn’t been quite as clear how the other elements with heavier atoms — atoms such as iodine, cesium, gold, lead, bismuth, uranium and so on — predominantly formed. Yes they can be formed in supernovas, but not so easily; and there seem to be more atoms of heavy elements around the universe than supernovas can explain. There are many supernovas in the history of the universe, but the efficiency for producing heavy chemical elements is just too low.

It was proposed some time ago that the mergers of neutron stars might be a suitable place to produce these heavy elements. Even those these mergers are rare, they might be much more efficient, because the nuclei of heavy elements contain lots of neutrons and, not surprisingly, a collision of two neutron stars would produce lots of neutrons in its debris, suitable perhaps for making these nuclei. A key indication that this is going on would be the following: if a neutron star merger could be identified using gravitational waves, and if its location could be determined using telescopes, then one would observe a pattern of light that would be characteristic of what is now called a “kilonova” explosion. Warning: I don’t yet know much about kilonovas and I may be leaving out important details. A kilonova is powered by the process of forming heavy elements; most of the nuclei produced are initially radioactive — i.e., unstable — and they break down by emitting high energy particles, including the particles of light (called photons) which are in the gamma ray and X-ray categories. The resulting characteristic glow would be expected to have a pattern of a certain type: it would be initially bright but would dim rapidly in visible light, with a long afterglow in infrared light. The reasons for this are complex, so let me set them aside for now. The important point is that this pattern was observed, confirming that a kilonova of this type occurred, and thus that, in this neutron star merger, enormous amounts of heavy elements were indeed produced. So we now have a lot of evidence, for the first time, that almost all the heavy chemical elements on and around our planet were formed in neutron star mergers. Again, we could not know this if we did not know that this was a neutron star merger, and that information comes only from the gravitational wave observation.

MISCELLANEOUS QUESTIONS

Did the merger of these two neutron stars result in a new black hole, a larger neutron star, or an unstable rapidly spinning neutron star that later collapsed into a black hole?

We don’t yet know, and maybe we won’t know. Some scientists involved appear to be leaning toward the possibility that a black hole was formed, but others seem to say the jury is out. I’m not sure what additional information can be obtained over time about this.

If the two neutron stars formed a black hole, why was there a kilonova? Why wasn’t everything sucked into the black hole?

Black holes aren’t vacuum cleaners; they pull things in via gravity just the same way that the Earth and Sun do, and don’t suck things in some unusual way. The only crucial thing about a black hole is that once you go in you can’t come out. But just as when trying to avoid hitting the Earth or Sun, you can avoid falling in if you orbit fast enough or if you’re flung outward before you reach the edge.

The point in a neutron star merger is that the forces at the moment of merger are so intense that one or both neutron stars are partially ripped apart. The material that is thrown outward in all directions, at an immense speed, somehow creates the bright, hot flash of gamma rays and eventually the kilonova glow from the newly formed atomic nuclei. Those details I don’t yet understand, but I know they have been carefully studied both with approximate equations and in computer simulations such as this one and this one. However, the accuracy of the simulations can only be confirmed through the detailed studies of a merger, such as the one just announced. It seems, from the data we’ve seen, that the simulations did a fairly good job. I’m sure they will be improved once they are compared with the recent data.

]]>72Matt Strasslerhttps://profmattstrassler.wordpress.comhttp://profmattstrassler.com/?p=98412017-10-16T16:28:20Z2017-10-16T15:10:02ZContinue reading →]]>Gravitational waves are now the most important new tool in the astronomer’s toolbox. Already they’ve been used to confirm that large black holes — with masses ten or more times that of the Sun — and mergers of these large black holes to form even larger ones, are not uncommon in the universe. Today it goes a big step further.

It’s long been known that neutron stars, remnants of collapsed stars that have exploded as supernovas, are common in the universe. And it’s been known almost as long that sometimes neutron stars travel in pairs. (In fact that’s how gravitational waves were first discovered, indirectly, back in the 1970s.) Stars often form in pairs, and sometimes both stars explode as supernovas, leaving their neutron star relics in orbit around one another. Neutron stars are small — just ten or so kilometers (miles) across. According to Einstein’s theory of gravity, a pair of stars should gradually lose energy by emitting gravitational waves into space, and slowly but surely the two objects should spiral in on one another. Eventually, after many millions or even billions of years, they collide and merge into a larger neutron star, or into a black hole. This collision does two things.

It makes some kind of brilliant flash of light — electromagnetic waves — whose details are only guessed at. Some of those electromagnetic waves will be in the form of visible light, while much of it will be in invisible forms, such as gamma rays.

It makes gravitational waves, whose details are easier to calculate and which are therefore distinctive, but couldn’t have been detected until LIGO and VIRGO started taking data, LIGO over the last couple of years, VIRGO over the last couple of months.

It’s possible that we’ve seen the light from neutron star mergers before, but no one could be sure. Wouldn’t it be great, then, if we could see gravitational waves AND electromagnetic waves from a neutron star merger? It would be a little like seeing the flash and hearing the sound from fireworks — seeing and hearing is better than either one separately, with each one clarifying the other. (Caution: scientists are often speaking as if detecting gravitational waves is like “hearing”. This is only an analogy, and a vague one! It’s not at all the same as acoustic waves that we can hear with our ears, for many reasons… so please don’t take it too literally.) If we could do both, we could learn about neutron stars and their properties in an entirely new way.

The merger was detected for more than a full minute… to be compared with black holes whose mergers can be detected for less than a second. It’s not exactly clear yet what happened at the end, however! Did the merged neutron stars form a black hole or a neutron star? The jury is out.

At almost exactly the moment at which the gravitational waves reached their peak, a blast of gamma rays — electromagnetic waves of very high frequencies — were detected by a different scientific team, the one from FERMI. FERMI detects gamma rays from the distant universe every day, and a two-second gamma-ray-burst is not unusual. And INTEGRAL, another gamma ray experiment, also detected it. The teams communicated within minutes. The FERMI and INTEGRAL gamma ray detectors can only indicate the rough region of the sky from which their gamma rays originate, and LIGO/VIRGO together also only give a rough region. But the scientists saw those regions overlapped. The evidence was clear. And with that, astronomy entered a new, highly anticipated phase.

Already this was a huge discovery. Brief gamma-ray bursts have been a mystery for years. One of the best guesses as to their origin has been neutron star mergers. Now the mystery is solved; that guess is apparently correct. (Or is it? Probably, but the gamma ray discovery is surprisingly dim, given how close it is. So there are still questions to ask.)

Also confirmed by the fact that these signals arrived within a couple of seconds of one another, after traveling for over 100 million years from the same source, is that, indeed, the speed of light and the speed of gravitational waves are exactly the same — both of them equal to the cosmic speed limit, just as Einstein’s theory of gravity predicts.

Next, these teams quickly told their astronomer friends to train their telescopes in the general area of the source. Dozens of telescopes, from every continent and from space, and looking for electromagnetic waves at a huge range of frequencies, pointed in that rough direction and scanned for anything unusual. (A big challenge: the object was near the Sun in the sky, so it could be viewed in darkness only for an hour each night!) Light was detected! At all frequencies! The object was very bright, making it easy to find the galaxy in which the merger took place. The brilliant glow was seen in gamma rays, ultraviolet light, infrared light, X-rays, and radio. (Neutrinos, particles that can serve as another way to observe distant explosions, were not detected this time.)

And with so much information, so much can be learned!

Most important, perhaps, is this: from the pattern of the spectrum of light, the conjecture seems to be confirmed that the mergers of neutron stars are important sources, perhaps the dominant one, for many of the heavy chemical elements — iodine, iridium, cesium, gold, platinum, and so on — that are forged in the intense heat of these collisions. It used to be thought that the same supernovas that form neutron stars in the first place were the most likely source. But now it seems that this second stage of neutron star life — merger, rather than birth — is just as important. That’s fascinating, because neutron star mergers are much more rare than the supernovas that form them. There’s a supernova in our Milky Way galaxy every century or so, but it’s tens of millenia or more between these “kilonovas”, created in neutron star mergers.

If there’s anything disappointing about this news, it’s this: almost everything that was observed by all these different experiments was predicted in advance. Sometimes it’s more important and useful when some of your predictions fail completely, because then you realize how much you have to learn. Apparently our understanding of gravity, of neutron stars, and of their mergers, and of all sorts of sources of electromagnetic radiation that are produced in those merges, is even better than we might have thought. But fortunately there are a few new puzzles. The X-rays were late; the gamma rays were dim… we’ll hear more about this shortly, as NASA is holding a second news conference.

Some highlights from the second news conference:

New information about neutron star interiors, which affects how large they are and therefore how exactly they merge, has been obtained

The first ever visual-light image of a gravitational wave source, from the Swope telescope, at the outskirts of a distant galaxy; the galaxy’s center is the blob of light, and the arrow points to the explosion.

The theoretical calculations for a kilonova explosion suggest that debris from the blast should rather quickly block the visual light, so the explosion dims quickly in visible light — but infrared light lasts much longer. The observations by the visible and infrared light telescopes confirm this aspect of the theory; and you can see evidence for that in the picture above, where four days later the bright spot is both much dimmer and much redder than when it was discovered.

Estimate: the total mass of the gold and platinum produced in this explosion is vastly larger than the mass of the Earth.

Estimate: these neutron stars were formed about 10 or so billion years ago. They’ve been orbiting each other for most of the universe’s history, and ended their lives just 130 million years ago, creating the blast we’ve so recently detected.

Big Puzzle: all of the previous gamma-ray bursts seen up to now have always had shone in ultraviolet light and X-rays as well as gamma rays. But X-rays didn’t show up this time, at least not initially. This was a big surprise. It took 9 days for the Chandra telescope to observe X-rays, too faint for any other X-ray telescope. Does this mean that the two neutron stars created a black hole, which then created a jet of matter that points not quite directly at us but off-axis, and shines by illuminating the matter in interstellar space? This had been suggested as a possibility twenty years ago, but this is the first time there’s been any evidence for it.

One more surprise: it took 16 days for radio waves from the source to be discovered, with the Very Large Array, the most powerful existing radio telescope. The radio emission has been growing brighter since then! As with the X-rays, this seems also to support the idea of an off-axis jet.

Nothing quite like this gamma-ray burst has been seen — or rather, recognized — before. When a gamma ray burst doesn’t have an X-ray component showing up right away, it simply looks odd and a bit mysterious. Its harder to observe than most bursts, because without a jet pointing right at us, its afterglow fades quickly. Moreover, a jet pointing at us is bright, so it blinds us to the more detailed and subtle features of the kilonova. But this time, LIGO/VIRGO told scientists that “Yes, this is a neutron star merger”, leading to detailed study from all electromagnetic frequencies, including patient study over many days of the X-rays and radio. In other cases those observations would have stopped after just a short time, and the whole story couldn’t have been properly interpreted.

Aside from the fact that this means that the VIRGO instrument actually works, which is great news, why is this a big deal? By adding a third gravitational wave detector, built by the VIRGO collaboration, to LIGO’s Washington and Louisiana detectors, the scientists involved in the search for gravitational waves now can determine fairly accurately the direction from which a detected gravitational wave signal is coming. And this allows them to do something new: to tell their astronomer colleagues roughly where to look in the sky, using ordinary telescopes, for some form of electromagnetic waves (perhaps visible light, gamma rays, or radio waves) that might have been produced by whatever created the gravitational waves.

The point is that with three detectors, one can triangulate. The gravitational waves travel for billions of years, traveling at the speed of light, and when they pass by, they are detected at both LIGO detectors and at VIRGO. But because it takes light a few thousandths of a second to travel the diameter of the Earth, the waves arrive at slightly different times at the LIGO Washington site, the LIGO Louisiana site, and the VIRGO site in Italy. The precise timing tells the scientists what direction the waves were traveling in, and therefore roughly where they came from. In a similar way, using the fact that sound travels at a known speed, the times that a gunshot is heard at multiple locations can be used by police to determine where the shot was fired.

You can see the impact in the picture below, which is an image of the sky drawn as a sphere, as if seen from outside the sky looking in. In previous detections of black hole mergers by LIGO’s two detectors, the scientists could only determine a large swath of sky where the observed merger might have occurred; those are the four colored regions that stretch far across the sky. But notice the green splotch at lower left. That’s the region of sky where the black hole merger announced today occurred. The fact that this region is many times smaller than the other four reflects what including VIRGO makes possible. It’s a small enough region that one can search using an appropriate telescope for something that is making visible light, or gamma rays, or radio waves.

While a black hole merger isn’t expected to be observable by other telescopes, and indeed nothing was observed by other telescopes this time, other events that LIGO might detect, such as a merger of two neutron stars, may create an observable effect. We can hope for such exciting news over the next year or two.

The top plot shows the number of X-rays (high-energy photons [particles of light]) coming from the sun, and that huge spike in the middle of the plot indicates a very powerful solar flare occurred about 24 hours ago. It should take about 2 days from the time of the flare for its other effects — the cloud of electrically-charged particles expelled from the Sun’s atmosphere — to arrive at Earth. The electrically-charged particles are what generate the auroras, when they are directed by Earth’s magnetic field to enter the Earth’s atmosphere near the Earth’s magnetic poles, where they crash into atoms in the upper atmosphere, exciting them and causing them to radiate visible light.

The flare was very powerful, but its cloud of particles didn’t head straight for Earth. We might get only a glancing blow. So we don’t know how big an effect to expect here on our planet. All we can do for now is be hopeful, and wait.

In any case, auroras borealis and australis are possible in the next day or so. Watch for the middle plot to go haywire, and for the bars in the lower plot to jump higher; then you know the time has arrived.

]]>3Matt Strasslerhttps://profmattstrassler.wordpress.comhttp://profmattstrassler.com/?p=96092017-09-07T16:53:11Z2017-08-18T12:30:54ZContinue reading →]]>Back in 1999 I saw a total solar eclipse in Europe, and it was a life-altering experience. I wrote about it back then, but was never entirely happy with the article. This week I’ve revised it. It could still benefit from some editing and revision (comments welcome), but I think it’s now a good read. It’s full of intellectual observations, but there are powerful emotions too.

After two years of dreaming, two months of planning, and two hours of packing, I drove to John F. Kennedy airport, took the shuttle to the Air France terminal, and checked in. I was brimming with excitement. In three days time, with a bit of luck, I would witness one the great spectacles that a human being can experience: a complete, utter and total eclipse of the Sun.

I had missed one eight years earlier. In July 1991, a total solar eclipse crossed over Baja California. I had thought seriously about driving the fourteen hundred miles from the San Francisco area, where I was a graduate student studying theoretical physics, to the very southern tip of the peninsula. But worried about my car’s ill health and scared by rumors of gasoline shortages in Baja, I chickened out. Four of my older colleagues, more worldly and more experienced, and supplied with a more reliable vehicle, drove down together. When they returned, exhilarated, they regaled us with stories of their magical adventure. Hearing their tales, I kicked myself for not going, and had been kicking myself ever since. Life is not so long that such opportunities can be rationalized or procrastinated away.

A total eclipse of the Sun is a event of mythic significance, so rare and extraordinary and unbelievable that it really ought to exist only in ancient legends, in epic poems, and in science fiction stories. There are other types of eclipses — partial and total eclipses of the Moon, in which the Earth blocks sunlight that normally illuminates the Moon, and various eclipses of the Sun in which the Moon blocks sunlight that normally illuminates the Earth. But total solar eclipses are in a class all their own. Only during the brief moments of totality does the Sun vanish altogether, leaving the shocked spectator in a suddenly darkened world, gazing uncomprehendingly at a black disk of nothingness.

Our species relies on daylight. Day is warm; day grows our food; day permits travel with a clear sense of what lies ahead. We are so fearful of the night — of what lurks there unseen, of the sounds that we cannot interpret. Horror films rely on this fear; demons and axe murderers are rarely found walking about in bright sunshine. Dark places are dangerous places; sudden unexpected darkness is worst of all. These are the conventions of cinema, born of our inmost psychology. But the Sun and the Moon are not actors projected on a screen. The terror is real.

It has been said that if the Earth were a member of a federation of a million planets, it would be a famous tourist attraction, because this home of ours would be the only one in the republic with such beautiful eclipses. For our skies are witness to a coincidence truly of cosmic proportions. It is a stunning accident that although the Sun is so immense that it could hold a million Earths, and the Moon so small that dozens could fit inside our planet, these two spheres, the brightest bodies in Earth’s skies, appear the same size. A faraway giant may seem no larger than a nearby child. And this perfect match of their sizes and distances makes our planet’s eclipses truly spectacular, visually and scientifically. They are described by witnesses as a sight of weird and unique beauty, a visual treasure completely unlike anything else a person will ever see, or even imagine.

But total solar eclipses are uncommon, occurring only once every year or two. Even worse, totality only occurs in a narrow band that sweeps across the Earth — often just across its oceans. Only a small fraction of the Earth sees a total eclipse in any century. And so these eclipses are precious; only the lucky, or the devoted, will experience one before they die.

In my own life, I’d certainly been more devoted than lucky. I knew it wasn’t wise to wait for the Moon’s shadow to find me by chance. Instead I was going on a journey to place myself in its path.

The biggest challenge in eclipse-chasing is the logistics. The area in which totality is visible is very long but very narrow. For my trip, in 1999, it was a long strip running west to east all across Europe, but only a hundred miles wide from north to south. A narrow zone crossing heavily populated areas is sure to attract a massive crowd, so finding hotels and transport can be difficult. Furthermore, although eclipses are precisely predictable, governed by the laws of gravity worked out by Isaac Newton himself, weather and human beings are far less dependable.

But I had a well-considered plan. I would travel by train to a small city east of Paris, where I had reserved a rental car. Keeping a close watch on the weather forecast, I would drive on back roads, avoiding clogged highways. I had no hotel reservations. It would have been pointless to make them for the night before the event, since it was well known that everything within two hours drive of the totality zone was booked solid. Moreover, I wanted the flexibility to adjust to the weather and couldn’t know in advance where I’d want to stay. So my idea was that on the night prior to the eclipse, I would drive to a good location in the path of the lunar shadow, and sleep in the back of my car. I had a sleeping bag with me to keep me warm, and enough lightweight clothing for the week — and not much else.

Oh, it was such a good plan, clean and simple, and that’s why my heart had so far to sink and my brain so ludicrous a calamity to contemplate when I checked my wallet, an hour before flight time, and saw a gaping black emptiness where my driver’s license was supposed to be. I was struck dumb. No license meant no car rental; no car meant no flexibility and no place to sleep. Sixteen years of driving and I had never lost it before; why, why, of all times, now, when it was to play a central role in a once-in-a-lifetime adventure?

I didn’t panic. I walked calmly back to the check-in counters, managed to get myself rescheduled for a flight on the following day, drove the three hours back to New Jersey, and started looking. It wasn’t in my car. Nor was it in the pile of unneeded items I’d removed from my wallet. Not in my suitcase, not under my bed, not in my office. As it was Sunday, I couldn’t get a replacement license. Hope dimmed, flickered, and went dark.

Deep breaths. Plan B?

I didn’t have a tent, and couldn’t easily have found one. But I did have a rain poncho, large enough to keep my sleeping bag off the ground. As long as it didn’t rain too hard, I could try, the night before the eclipse, to find a place to camp outdoors; with luck I’d find lodging for the other nights. I doubted this would be legal, but I was willing to take the chance. But what about my suitcase? I couldn’t carry that around with me into the wilderness. Fortunately, I knew a solution. For a year after college, I had studied music in France, and had often gone sightseeing by rail. On those trips I had commonly made use of the ubiquitous lockers at the train stations, leaving some luggage while I explored the nearby town. As for flexibility of location, that was unrecoverable; the big downside of Plan B was that I could no longer adjust to the weather. I’d just have to be lucky. I comforted myself with the thought that the worst that could happen to me would be a week of eating French food.

So the next day, carrying the additional weight of a poncho and an umbrella, but having in compensation discarded all inessential clothing and tourist information, I headed back to the airport, this time by bus. Without further misadventures, I was soon being carried across the Atlantic.

As usual I struggled to nap amid the loud silence of a night flight. But my sleeplessness was rewarded with one of those good omens that makes you think that you must be doing the right thing. As we approached the European coastline, and I gazed sleepily out my window, I suddenly saw a bright glowing light. It was the rising white tip of the thin crescent Moon.

Solar eclipses occur at New Moon, always. This is nothing but simple geometry; the Moon must place itself exactly between the Sun and the Earth to cause an eclipse, and that means the half of the Moon that faces us must be in shadow. (At Full Moon, the opposite is true; the Earth is between the Sun and the Moon, so the half of the Moon that faces us is in full sunlight. That’s when lunar eclipses can occur.) And just before a New Moon, the Moon is close to the Sun’s location in the sky. It becomes visible, as the Earth turns, just before the Sun does, rising as a morning crescent shortly before sunrise. (Similarly, we get an evening crescent just after a New Moon.)

There, out over the vast Atlantic, from a dark ocean of water into a dark sea of stars, rose the delicate thin slip of Luna the lover, on her way to her mystical rendezvous with Sol. Her crescent smiled at me and winked a greeting. I smiled back, and whispered, “see you in two days…” For totality is not merely the only time you can look straight at the Sun and see its crown. It is the only time you can see the New Moon.

We landed in Paris at 6:30 Monday morning, E-day-minus-two. I headed straight to the airport train station, and poured over rail maps and my road maps trying to guess a good location to use as a base. Eventually I chose a medium-sized town with the name Charleville-Mezieres. It was on the northern edge of the totality zone, at the end of a large spoke of the Paris-centered rail system, and was far enough from Paris, Brussels, and all large German towns that I suspected it might escape the worst of the crowds. It would then be easy, the night before the eclipse, to take a train back into the center of the zone, where totality would last the longest.

Two hours later I was in the Paris-East rail station and had purchased my ticket for Charleville-Mezieres. With ninety minutes to wait, I wandered around the station. It was evident that France had gone eclipse-happy. Every magazine had a cover story; every newspaper had a special insert; signs concerning the event were everywhere. Many of the magazines carried free eclipse glasses, with a black opaque metallic material for lenses that only the Sun can penetrate. Warnings against looking at the Sun without them were to be found on every newspaper front page. I soon learned that there had been a dreadful scandal in which a widely distributed shipment of imported glasses was discovered to be dangerously defective, leading the government to make a hurried and desperate attempt to recall them. There were also many leaflets advertising planned events in towns lying in the totality zone, and information about extra trains that would be running. A chaotic rush out of Paris was clearly expected.

Before noon I was on a train heading through the Paris suburbs into the farmlands of the Champagne region. The rocking of the train put me right to sleep, but the shrieking children halfway up the rail car quickly ended my nap. I watched the lovely sunlit French countryside as it rolled by. The Sun was by now well overhead — or rather, the Earth had rotated so that France was nearly facing the Sun head on. Sometimes, when the train banked on a turn, the light nearly blinded me, and I had to close my eyes.

With my eyelids shut, I thought about how I’d managed, over decades, to avoid ever once accidentally staring at the Sun for even a second… and about how almost every animal with eyes manages to do this during its entire life. It’s quite a feat, when you think about it. But it’s essential, of course. The Sun’s ferocious blaze is even worse than it appears, for it contains more than just visible light. It also radiates light too violet for us to see — ultraviolet — which is powerful enough to destroy our vision. Any animal lacking instincts powerful enough to keep its eyes off the Sun will go blind, soon to starve or be eaten. But humans are in danger during solar eclipses, because our intense curiosity can make us ignore our instincts. Many of us will suffer permanent eye damage, not understanding when and how it is safe to look at the Sun… which is almost, but not quite, never.

In fact the only time it is safe to look with the naked eye is during totality, when the Sun’s disk is completely blocked by the New Moon, and the world is dark. Then, and only then, can one see that the Sun is not a sphere, and that it has a sort of atmosphere, immense and usually unseen.

At the heart of the Sun, and source of its awesome power, is its nuclear furnace, nearly thirty million degrees hot and nearly five billion years old. All that heat gradually filters and boils out of the Sun’s core toward its visible surface, which is a mere six thousand degrees… still white-hot. Outside this region is a large irregular halo of material that is normally too dim to see against the blinding disk. The inner part of that halo is called the chromosphere; there, giant eruptions called “prominences” loop outward into space. The outer part of the halo is the “corona”, Latin for “crown.” The opportunity to see the Sun’s corona is one of the main reasons to seek totality.

Still very drowsy, but in a good mood, I arrived in Charleville. Wanting to leave my bags in the station while I looked for a hotel room, I searched for the luggage lockers. After three tiring trips around the station, I asked at a ticket booth. “Oh,” said the woman behind the desk, “we haven’t had them available since the Algerian terrorism of a few years ago.”

I gulped. This threatened plan B, for what was I to do with my luggage on eclipse day? I certainly couldn’t walk out into the French countryside looking for a place to camp while carrying a full suitcase and a sleeping bag! And even the present problem of looking for a hotel would be daunting. The woman behind the desk was sympathetic, but her only suggestion was to try one of the hotels near the station. Since the tourist information office was a mile away, it seemed the only good option, and I lugged my bags across the street.

Here, finally, luck smiled. The very first place I stopped at had a room for that night, reasonably priced and perfectly clean, if spartan. It was also available the night after the eclipse. My choice of Charleville had been wise. Unfortunately, even here, Eclipse Eve — Tuesday evening — was as bad as I imagined. The hoteliere assured me that all of Charleville was booked (and my later attempts to find a room, even a last-minute cancellation, proved fruitless.) Still, she she was happy for me to leave my luggage at the hotel while I tramped through the French countryside. Thus was Plan B saved.

Somewhat relieved, I wandered around the town. Charleville is not unattractive, and the orange sandstone 16th century architecture of its central square is very pleasing to the eye. By dusk I was exhausted and collapsed on my bed. I slept long and deep, and awoke refreshed. I took a short sightseeing trip by train, ate a delicious lunch, and tried one more time to find a room in Charleville for Eclipse Eve. Failing once again, I resolved to camp in the heart of the totality zone.

But where? I had several criteria in mind. For the eclipse, I wanted to be far from any large town or highway, so that streetlights, often automatically triggered by darkness, would not spoil the experience. Also I wanted hills and farmland; I wanted to be at a summit, with no trees nearby, in order to have the best possible view. It didn’t take long to decide on a location. About five miles south of the unassuming town of Rethel, rebuilt after total destruction in the first world war, my map showed a high hill. It seemed perfect.

Fortunately, I learned just in time that this same high hill had attracted the attention of the local authorities, and they had decided to designate this very place the “official viewing site” in the region. A hundred thousand people were expected to descend on Rethel and take shuttles from the town to the “site.” Clearly this was not where I wanted to be!

So instead, when I arrived in Rethel, I walked in another direction. I aimed for an area a few miles west of town, quiet hilly farmland.

Yet again, my luck seemed to be on the wane. By four it was drizzling, and by five it was raining. Darkness would settle at around eight, and I had little time to find a site for unobtrusive camping, much less a dry one. The rain stopped, restarted, hesitated, spat, but refused to go away. An unending mass of rain clouds could be seen heading toward me from the west. I had hoped to use trees for some shelter against rain, but now the trees were drenched and dripping, even worse than the rain itself.

Still completely unsure what I would do, I continued walking into the evening. I must have cut a very odd figure, carrying an open umbrella, a sleeping bag, and a small black backpack. I took a break in a village square, taking shelter at a church’s side door, where I munched on French bread and cheese. Maybe one of these farmers would let me sleep in a dry spot in his barn, I thought to myself. But I still hadn’t reached the hills I was aiming for, so I kept walking.

After another mile, I came to a hilltop with a dirt farm track crossing the road. There, just off the road to the right, was a large piece of farm machinery. And underneath it, a large, flat, sheltered spot. Hideous, but I could sleep there. Since it wasn’t quite nightfall yet and I could see a hill on the other side of the road along the same track, one which looked like it might be good for watching the eclipse, I took a few minutes to explore it. There I found another piece of farm equipment, also with a sheltered underbelly. This one was much further from the road, looked unused, and presumably offered both safer and quieter shelter. It was sitting just off the dirt track in a fallow field. The field was of thick, sticky, almost hard mud, the kind you don’t slip in and which doesn’t ooze but which gloms onto the sides of your shoe.

And so it was that Eclipse Eve found me spreading my poncho in a friendly unknown farmer’s field, twisting my body so as not to hit my head on the metal bars of my shelter, carefully unwrapping my sleeping bag and removing my shoes so as not to cover everything in mud, brushing my teeth in bottled water, and bedding down for the night. The whole scene was so absurd that I found myself sporting a slightly manic grin and giggling. But still, I was satisfied. Despite the odds, I was in the zone at the appointed time; when I awoke the next morning I would be scarcely two miles from my final destination. If the clouds were against me, so be it. I had done my part.

I slept pretty well, considering both my excitement and the uneven ground. At daybreak I was surrounded by fog, but by 8 a.m.~the fog was lifting, revealing a few spots of blue sky amid low clouds. My choice of shelter was also confirmed; my sleeping bag was dry, and across the road the other piece of machinery I had considered was already in use.

I packed up and started walking west again. The weather seemed uncertain, with three layers of clouds — low stratus, medium cumulus, and high cirrus — crossing over each other. Blue patches would appear, then close up. I trudged to the base of my chosen hill, then followed another dirt track to the top, where I was graced with a lovely view. The rolling paysage of fertile France stretched before me, blotched here and there with sunshine. Again I had chosen well, better than I realized, as it turned out, for I was not alone on the hill. A Belgian couple had chosen it too — and they had a car…

There I waited. The minutes ticked by. The temperature fluctuated, and the fields changed color, as the Sun played hide and seek. I didn’t need these reminders of the Sun’s importance — that without its heat the Earth would freeze, and without its light, plants would not grow and the cycle of life would quickly end. I thought about how pre-scientific cultures had viewed the Sun. In cultures and religions around the world, the blazing disk has often been attributed divine power and regal authority. And why not? In the past century, we’ve finally learned what the Sun is made from and why it shines. But we are no less in awe than our ancestors, for the Sun is much larger, much older, and much more powerful than most of them imagined.

For a while, I listened to the radio. Crowds were assembling across Europe. Special events — concerts, art shows, contests — were taking place, organized by towns in the zone to coincide with the eclipse. This was hardly surprising. All those tourists had come for totality. But totality is brief, never more than a handful of minutes. It’s the luck of geometry, the details of the orbits of the Earth and Moon, that set its duration. For my eclipse, the Moon’s shadow was only about a hundred miles wide. Racing along at three thousand miles per hour, it would darken any one location for at most two minutes. Now if a million people are expected to descend on your town for a two-minute event, I suppose it is a good idea to give them something else to do while they wait. And of course, the French cultural establishment loves this kind of opportunity. Multimedia events are their specialty, and they often give commissions to contemporary artists. I was particularly amused to discover later that an old acquaintance of mine — I met him in 1987 at the composers’ entrance exams for the Paris Conservatory — had been commissioned to write an orchestral piece, called “Eclipse,” for the festival in the large city of Reims. It was performed just before the moment of darkness.

Finally, around 11:30, the eclipse began. The Moon nibbled a tiny notch out of the sun. I looked at it briefly through my eclipse glasses, and felt the first butterflies of anticipation. The Belgian couple, in their late fourties, came up to the top of the hill and stood alongside me. They were Flemish, but the man spoke French, and we chatted for a while. It turned out he was a scientist also, and had spent some time in the United States, so we had plenty to talk about. But our discussion kept turning to the clouds, which showed no signs of dissipating. The Sun was often veiled by thin cirrus or completely hidden by thick cumulus. We kept a nervous watch.

Time crawled as the Moon inched across the brilliant disk. It passed the midway point and the Sun became a crescent. With only twenty minutes before totality, my Belgian friends conversed in Dutch. The man turned to me. “We have decided to drive toward that hole in the clouds back to the east,” he said in French. “It’s really not looking so good here. Do you want to come with us?” I paused to think. How far away was that hole? Would we end up back at the town? Would we get caught in traffic? Would we end up somewhere low? What were my chances if I stayed where I was? I hesitated, unsure. If I went with them, I was subject to their whims, not my own. But after looking at the oncoming clouds one more time, I decided my present location was not favorable. I joined them.

We descended the dirt track and turned left onto the road I’d taken so long to walk. It was completely empty. We kept one eye on where we were going and five eyes on the sky. After two miles, the crescent sun became visible through a large gap in the low clouds. There were still high thin clouds slightly veiling it, but the sky around it was a pale blue. We went a bit further, and then stopped… at the very same dirt track where I had slept the night before. A line of ten or fifteen cars now stretched along it, but there was plenty of room for our vehicle.

By now, with ten minutes to go, the light was beginning to change. When only five percent of the Sun remains, your eye can really tell. The blues become deeper, the whites become milkier, and everything is more subdued. Also it becomes noticeably cooler. I’d seen this light before, in New Mexico in 1994. I had gone there to watch an “annular” eclipse of the Sun. An annular eclipse occurs when the Moon passes directly in front of the Sun but is just a bit too far away from the Earth for its shadow to reach the ground. In such an eclipse, the Moon fails to completely block the Sun; a narrow ringlet, or “annulus”, often called the “ring of fire,” remains visible. That day I watched from a mountain top, site of several telescopes, in nearly clear skies. But imagine the dismay of the spectators as the four-and-a-half minutes of annularity were blocked by a five-minute cloud! Fortunately there was a bright spot. For a brief instant — no more than three seconds — the cloud became thin, and a perfect circle of light shone through, too dim to penetrate eclipse glasses but visible with the naked eye… a veiled, surreal vision.

On the dirt track in the middle of French fields, we started counting down the minutes. There was more and more tension in the air. I put faster speed film into my camera. The light became still milkier, and as the crescent became a fingernail, all eyes were focused either on the Sun itself or on a small but thick and dangerous-looking cloud heading straight for it. Except mine. I didn’t care if I saw the last dot of sunlight disappear. What I wanted to watch was the coming of Moon-shadow.

One of my motivations for seeking a hill was that I wanted to observe the approach of darkness. Three thousand miles an hour is just under a mile per second, so if one had a view extending out five miles or so, I thought, one could really see the edge coming. I expected it would be much like watching the shadow of a cloud coming toward me, with the darkness sweeping along the ground, only much darker and faster. I looked to the west and waited for the drama to unfold.

And it did, but it was not what I was expecting. Even though observing the shadow is a common thing for eclipse watchers to do, nothing I had ever read about eclipses prepared me in the slightest for what I was about to witness. I’ve never seen it photographed, or even described. Maybe it was an effect of all the clouds around us. Or maybe others, just as I do, find it difficult to convey.

For how can one relate the sight of daylight sliding swiftly, like an sigh, to deep twilight? of the western sky, seen through scattered clouds, changing seamlessly and inexorably from blue to pink to slate gray to the last yellow of sunset? of colors rising up out of the horizon and spreading across the sky like water from a broken dyke flooding onto a field?

I cannot find the right combination of words to capture the sense of being swept up, of being overwhelmed, of being transfixed with awe, as one might be before the summoning of a great wave or a great wind by the command of a god, yet all in utter silence and great beauty. Reliving it as I write this brings a tear. In the end I have nothing to compare it to.

The great metamorphosis passed. The light stabilized. Shaken, I looked up.

And quickly looked away. I had seen a near-disk of darkness, the fuzzy whiteness of the corona, and some bright dots around the disk’s edge, one especially bright where the Sun still clearly shone through. Accidentally I had seen with my naked eyes the “diamond ring,” a moment when the last brilliant drop of Sun and the glistening corona are simultaneously visible. It’s not safe to look at. I glanced again. Still several bright dots. I glanced again. Still there — but the Sun had to be covered by now…

So I looked longer, and realized that the Sun was indeed covered, that those bright dots were there to stay. There it was. The eclipsed Sun, or rather, the dark disk of the New Moon, surrounded by the Sun’s crown, studded at its edge with seven bright pink jewels. It was bizarre, awe-inspiring, a spooky hallucination. It shimmered.

The Sun’s corona didn’t really resemble what I had seen in photographs, and I could immediately see why. The corona looked as though it were made of glistening white wispy hair, billowing outward like a mop of whiskers. It gleamed with a celestial light, a shine resembling that of well-lit tinsel. No camera could capture that glow, no photograph reproduce it.

But the greatest, most delightful surprise was the seven beautiful gems. I knew they had to be the great eruptions on the surface of the Sun, prominences, huge magnetic storms larger than our planet and more violent than anything else in the solar system. However, nobody ever told me they were bright pink! I always assumed they were orange (silly of me, since the whole Sun looks orange if you look at it through an orange filter, which the photographs always do.) They were arranged almost symmetrically around the sun, with one of them actually well separated from its surface and halfway out into the lovely soft filaments of the corona. I explored them with my binoculars. The colors, the glistening timbre, the rich detail, it is a visual delight. The scene is living, vibrant, delicate and soft; by comparison, all the photographs and films seem dry, flat, deadened.

I was surprised at my calm. After the great rush of the shadow, the stasis of totality had caught me off guard. Around me it was much lighter than I had expected. The sense was of late twilight, with a deep blue-purple sky; yet it was still bright enough to read by. The yellow light of late sunset stretched all the way around the horizon. The planet Venus was visible, but no stars peeked through the clouds. Perhaps longer eclipses have darker skies, a larger Moon-shadow putting daylight further away.

I had scarcely had time to absorb all of this when, just at the halfway point of totality, the dangerous-looking cumulus cloud finally arrived, and blotted out the view. A groan, but only a half-hearted one, emerged from the spectators; after all we’d seen what we’d come to see. I took in the colors emanating from the different parts of the sky, and then looked west again, waiting for the light to return. A thin red glow touched the horizon. I waited. Suddenly the red began to grow furiously. I yelled “Il revient!” — it is returning! — and then watched in awe as the reds became pinks, swarmed over us, turned yellow-white…

And then… it was daylight again. Normality, or a slightly muted version of it. The magical show was over, heavenly love had been consummated, we who had traveled far had been rewarded. The weather had been kind to us. There was a pause as we savored the experience, and waited for our brains to resume functioning. Then congratulations were passed around as people shook hands and hugged each other. I thanked my Belgian friends, who like me were smiling broadly. They offered me a ride back to town. I almost accepted, but stopped short, and instead thanked them again and told them I somehow wanted to be outside for a while longer. We exchanged addresses, said goodbyes, they drove off.

I started retracing my steps from the previous evening. As I walked back to the town of Rethel in the returning sunshine, the immensity of what I had seen began gradually to make its way through my skin into my blood, making me teary-eyed. I thought about myself, a scientist, educated and knowledgeable about the events that had just taken place, and tried to imagine what would have happened to me today if I had not had
that knowledge and had found myself, unexpectedly, in the Moon’s shadow.

It was not difficult; I had only to imagine what I would feel if the sky suddenly, without any warning, turned a fiery red instead of blue and began to howl. It would have been a living nightmare. The terror that I would have felt would have penetrated my bones. I would have fallen on my knees in panic; I would have screamed and wept; I would have called on every deity I knew and others I didn’t know for help; I would have despaired; I would have thought death or hell had come; I would have assumed my life was about to end. The two minutes of darkness, filled with the screams and cries of my neighbors, would have been timeless, maddening. When the Sun just as suddenly returned, I would have collapsed onto the ground with relief, profusely and weepingly thanking all of the deities for restoring the world to its former condition, and would have rushed home to relatives and friends, hoping to find some comfort and solace.

I would have sought explanations. I would have been willing to consider anything: dragons eating the Sun, spirits seeking to punish our village or country for its transgressions, evil and spiteful monsters trying to freeze the Earth, gods warning us of terrible things to come in future. But above all, I could never, never have imagined that this brief spine-chilling extinction and transformation of the Sun was a natural phenomenon. Nothing so spectacular and sudden and horrifying could have been the work of mere matter. It would once and for all have convinced me of the existence of creatures greater and more powerful than human beings, if I had previously had any doubt.

And I would have been forever changed. No longer could I have entirely trusted the regularity of days and nights, of seasons, of years. For the rest of my life I would have always found myself glancing at the sky, wanting to make sure that all, for the moment, was well. For if the Sun could suddenly vanish for two minutes, perhaps the next time it could vanish for two hours, or two days… or two centuries. Or forever.

I pondered the impact that eclipses, both solar and lunar, have had throughout human history. They have shaped civilizations. Wars and slaughters were begun and ended on their appearance; they sent ordinary people to their deaths as appeasement sacrifices; new gods and legends were invoked to give meaning to them. The need to predict them, and the coincidences which made their prediction possible, helped give birth to astronomy as a mathematically precise science, in China, in Greece, in modern Europe — developments without which my profession, and even my entire technologically-based culture, might not exist.

It was an hour’s walk to Rethel, but that afternoon it was a long journey. It took me across the globe to nations ancient and distant. By the time I reached the town, I’d communed with my ancestors, reconsidered human history, and examined anew my tiny place in the universe. If I’d been a bit calm during totality itself, I wasn’t anymore. What I’d seen was gradually filtering, with great potency, into my soul.

I took the train back to Charleville, and slept dreamlessly. The next two days were an opportunity to unwind, to explore, and to eat well. On my last evening I returned to Paris to visit my old haunts. I managed to sneak into the courtyard of the apartment house where I had had a one-room garret up five flights of stairs, with its spartan furnishings and its one window that looked over the roofs of Paris to the Eiffel Tower. I wandered past the old Music Conservatory, since moved to the northeast corner of town, and past the bookstore where I bought so much music. My favorite bakery was still open.

That night I slept in an airport hotel, and the next day flew happily home to the American continent. I never did find my driver’s license.

But psychological closure came already on the day following the eclipse. I spent that day in Laon, a small city perched magnificently atop a rocky hill that rises vertically out of the French plains. I wandered its streets and visited its sights — an attractive church, old houses, pleasant old alleyways, ancient walls and gates. As evening approached I began walking about, looking for a restaurant, and I came to the northwestern edge of town overlooking the new city and the countryside beyond. The clouds had parted, and the Sun, looking large and dull red, was low in the sky. I leaned on the city wall and watched as the turning Earth carried me, and Laon, and all of France, at hundreds of miles an hour, intent on placing itself between me and the Sun. Yet another type of solar eclipse, one we call “sunset.”

The ruddy disk touched the horizon. I remembered the wispy white mane and the brilliant pink jewels. In my mind the Sun had always been grand and powerful, life-giver and taker, essential and dangerous. It could blind, burn, and kill. I respected it, was impressed and awed by it, gave thanks for it, swore at it, feared it. But in the strange light of totality, I had seen beyond its unforgiving, blazing sphere, and glimpsed a softer side of the Sun. With its feathery hair blowing in a dark sky, it had seemed delicate, even vulnerable. It is, I thought to myself, as mortal as we.

The distant French hills rose across its face. As it waned, I found myself feeling a warmth, even a tenderness — affection for this giant glowing ball of hydrogen, this protector of our planet, this lonely beacon in a vast emptiness… the only star you and I will ever know.

]]>19Matt Strasslerhttps://profmattstrassler.wordpress.comhttp://profmattstrassler.com/?p=95362017-07-16T21:20:26Z2017-07-16T21:09:15ZContinue reading →]]>As forecast, the cloud of particles from Friday’s solar flare (the “coronal mass emission”, or “CME”) arrived at our planet a few hours after my last post, early in the morning New York time. If you’d like to know how I knew that it had reached Earth, and how I know what’s going on now, scroll down to the end of this post and I’ll show you the data I was following, which is publicly available at all times.

So far the resulting auroras have stayed fairly far north, and so I haven’t seen any — though they were apparently seen last night in Washington and Wyoming, and presumably easily seen in Canada and Alaska. [Caution: sometimes when people say they’ve been “seen”, they don’t quite mean that; I often see lovely photos of aurora that were only visible to a medium-exposure camera shot, not to the naked eye.] Or rather, I should say that the auroras have stayed fairly close to the Earth’s poles; they were also seen in New Zealand.

Russia and Europe have a good opportunity this evening. As for the U.S.? The storm in the Earth’s magnetic field is still going on, so tonight is still a definite possibility for northern states. Keep an eye out! Look for what is usually a white or green-hued glow, often in swathes or in stripes pointing up from the northern horizon, or even overhead if you’re lucky. The stripes can move around quite rapidly.

Now, here’s how I knew all this. I’m no expert on auroras; that’s not my scientific field at all. But the U.S. Space Weather Prediction Center at the National Oceanic and Atmospheric Administration, which needs to monitor conditions in space in case they should threaten civilian and military satellites or even installations on the ground, provides a wonderful website with lots of relevant data.

The first image on the site provides the space weather overview; a screenshot from the present is shown below, with my annotations. The upper graph indicates a blast of x-rays (a form of light not visible to the human eye) which is generated when the solar flare, the magnetically-driven explosion on the sun, first occurs. Then the slower cloud of particles (protons, electrons, and other atomic nuclei, all of which have mass and therefore can’t travel at light’s speed) takes a couple of days to reach Earth. It’s arrival is shown by the sudden jump in the middle graph. Finally, the lower graph measures how active the Earth’s magnetic field is. The only problem with that plot is it tends to be three hours out of date, so beware of that! A “Kp index” of 5 shows significant activity; 6 means that auroras are likely to be moving away from the poles, and 7 or 8 mean that the chances in a place like the north half of the United States are pretty good. So far, 6 has been the maximum generated by the current flare, but things can fluctuate a little, so 6 or 7 might occur tonight. Keep an eye on that lower plot; if it drops back down to 4, forget it, but it it’s up at 7, take a look for sure!

Also on the site is data from the ACE satellite. This satellite sits 950 thousand miles [1.5 million kilometers] from Earth, between Earth and the Sun, which is 93 million miles [150 million kilometers] away. At that vantage point, it gives us (and our other satellites) a little early warning, of up to an hour, before the cloud of slow particles from a solar flare arrives. That provides enough lead-time to turn off critical equipment that might otherwise be damaged. And you can see, in the plot below, how at a certain time in the last twenty-four hours the readings from the satellite, which had been tepid before, suddenly started fluctuating wildly. That was the signal that the flare had struck the satellite, and would arrive shortly at our location.

It’s a wonderful feature of the information revolution that you can get all this scientific data yourself, and not wait around hoping for a reporter or blogger to process it for you. None of this was available when I was a child, and I missed many a sky show. A big thank you to NOAA, and to the U.S. taxpayers who make their work possible.

]]>4Matt Strasslerhttps://profmattstrassler.wordpress.comhttp://profmattstrassler.com/?p=95152017-07-15T21:54:56Z2017-07-15T21:54:56ZContinue reading →]]>The Sun is busy this summer. The upcoming eclipse on August 21 will turn day into deep twilight and transfix millions across the United States. But before we get there, we may, if we’re lucky, see darkness transformed into color and light.

On Friday July 14th, a giant sunspot in our Sun’s upper regions, easily visible if you project the Sun’s image onto a wall, generated a powerful flare. A solar flare is a sort of magnetically powered explosion; it produces powerful electromagnetic waves and often, as in this case, blows a large quantity of subatomic particles from the Sun’s corona. The latter is called a “coronal mass ejection.” It appears that the cloud of particles from Friday’s flare is large, and headed more or less straight for the Earth.

Light, visible and otherwise, is an electromagnetic wave, and so the electromagnetic waves generated in the flare — mostly ultraviolet light and X-rays — travel through space at the speed of light, arriving at the Earth in eight and a half minutes. They cause effects in the Earth’s upper atmosphere that can disrupt radio communications, or worse. That’s another story.

But the cloud of subatomic particles from the coronal mass ejection travels a few hundred times slower than light, and it takes it about two or three days to reach the Earth. The wait is on.

Bottom line: a huge number of high-energy subatomic particles may arrive in the next 24 to 48 hours. If and when they do, the electrically charged particles among them will be trapped in, and shepherded by, the Earth’s magnetic field, which will drive them spiraling into the atmosphere close to the Earth’s polar regions. And when they hit the atmosphere, they’ll strike atoms of nitrogen and oxygen, which in turn will glow. Aurora Borealis, Northern Lights.

So if you live in the upper northern hemisphere, including Europe, Canada and much of the United States, keep your eyes turned to the north (and to the south if you’re in Australia or southern South America) over the next couple of nights. Dark skies may be crucial; the glow may be very faint.

You can also keep abreast of the situation, as I will, using NOAA data, available for instance at

The plot on the upper left of that website, an example of which is reproduced below, shows three types of data. The top graph shows the amount of X-rays impacting the atmosphere; the big jump on the 14th is Friday’s flare. And if and when the Earth’s magnetic field goes nuts and auroras begin, the bottom plot will show the so-called “Kp Index” climbing to 5, 6, or hopefully 7 or 8. When the index gets that high, there’s a much greater chance of seeing auroras much further away from the poles than usual.

Keep an eye also on the data from the ACE satellite, lower down on the website; it’s placed to give Earth an early warning, so when its data gets busy, you’ll know the cloud of particles is not far away.